1. Field of the Invention
The invention relates to the field of dynamic magnetic information storage or retrieval. More particularly, the invention relates to the field of magnetic tape heads. In still greater particularity, the invention relates to magnetic tape head configuration. By way of further characterization, but not by way of limitation thereto, the invention includes a split top pole tape head for writing unique servo patterns.
2. Description of the Related Art
Magnetic tape recording has been utilized for many years to record voice and data information. For information storage and retrieval, magnetic tape has proven especially reliable, cost efficient and easy to use. In an effort to make magnetic tape even more useful and cost effective, there have been attempts to store more information per given width and length of tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, this increase in the number of data tracks results in those tracks being more densely packed onto the tape. As the data tracks are more closely spaced, precise positioning of the tape with respect to the tape head becomes more critical as errors may be more easily introduced into the reading or writing of data. The tape to tape head positioning may be affected by variations in the tape or tape head, tape movement caused by air flow, temperature, humidity, tape shrinkage, and other factors, especially at the outside edges of the tape.
In order to increase data track accuracy, servo tracks have been employed to provide a reference point to maintain correct positioning of the tape with respect to the tape head. One or more servo tracks may be used depending upon the number of heads used to record and retrieve data from the multiple data tracks on the tape. Referring to FIG. 1, a one-half inch wide magnetic tape 11 may contain up to 288 or more data tracks on multiple data bands 12. With such a large number of data tracks it may be desirable to include up to five or more servo bands 13 to improve data read and write function performance. Servo bands 13 may utilize various patterns or frequency regions to allow precise tape to tape head positioning. Servo bands 13 may include one or more servo tracks.
An example of a servo pattern is shown in FIG. 2. This servo pattern is more specifically described in copending U.S. patent application entitled "Tape Servo Pattern with Enhanced Synchronization Properties", Ser. No. 08/804,445, now U.S. Pat. No. 6,023,385, filed on the same date herewith and assigned to the same assignee as the present application. That Application is hereby incorporated by reference as if fully set forth herein. Referring to FIG. 2, multiple recorded patterns are included as well as erased areas. In this pattern, a wide synchronization signal is denoted as a synchronization frequency region 14. A second, different frequency area 15 is written following region 14. A plurality of erase patterns 16 are written in a precise configuration such as parallelograms over portions of area 15.
During operation of the tape drive, the location of the tape head relative to the tape is controlled by servo readers which monitor the output signal when the reader is positioned at the edge of erase patterns 16. The position of erase patterns 16 relative to one another is very critical because edges are used to position the head relative to the tape. That is, during read or write operations the head may be moved relative to the tape to allow accurate reading or writing of the data tracks. In addition to the positioning of erase patterns 16, uniformity of the signal within the different frequency area 15 and erase patterns 16 is critical. Improper location of the erase patterns 16 or variations in the readback amplitude of the servo pattern may result in tracks of recorded data being improperly positioned on the tape or in erroneous reading of recorded data tracks.
In order to write a servo pattern as described above and shown in FIG. 2, the write heads for the servo pattern must be closely matched and tightly specified in order to achieve acceptable performance. The servo pattern is written in a manufacturing environment and thus the servo pattern must be written in one pass of the tape to be cost effective. It is also important to have feedback to the write driver circuitry to allow verification of the correctness of the servo pattern being written. Of course, ease of manufacture of the servo write and read head itself is also of primary importance while maintaining performance parameters.
Thin film write heads are being used today to write servo patterns. A thin film write head is processed primarily in a clean room atmosphere using thin film deposition and photolithography techniques with minimal mechanical processing. This type head is an advancement over previous write head fabrication techniques that had much more intensive mechanical processing. The difference is most notable in the definition of write poles. For a thin film write head the write pole geometry is defined using photolithography processes which allow a tight control over the critical write pole width, in contrast to previous write head designs where the write track width was defined mechanically with less accuracy. Thin film write heads can also be better optimized for recording servo patterns at high recording density and tape speed on high coercivity media.
It may be possible to utilize existing thin film tape head manufacturing techniques to allow multiple erase heads to be placed adjacent to one another, one for each erase band 16 in a servo pattern. However, such multiple tape head configurations would require severe design compromises from the performance of the tape head and/or the manufacturability of the head itself. Referring to FIG. 3 a portion of a cross-section of a prior art thin film write track of a tape head is shown. The tape head includes a closure 17 and a substrate 18 made of aluminum titanium carbide (AlTiC) or other suitable material. Between closure 17 and substrate 18 are a plurality of layers deposited upon substrate 18 in a clean room. These layers include an alumina underlayer 19 deposited upon substrate 18 and a planar bottom pole 20 deposited upon alumina underlayer 19. A gap layer 21 made of an insulating material such as alumina is deposited on planar bottom pole 20. A coil structure 22 is deposited upon gap layer 21 in accordance with conventional practice. Coil structure 22 comprises a series of deposited layers of baked photoresist or other suitable material with a coil positioned therebetween as is known in the art. A top pole 40 has a resulting bulged shape. Because the topography of these deposited layers is nonplanar it is necessary to deposit a planarizing alumina overcoat layer 30 over the layers already deposited. Thick overcoat layer 30 is lapped flat to provide a planar surface for receiving closure 17. The magnetic tape passes over a tape bearing surface 10 for being read from, and written to, by the tape head.
Referring to FIG. 4, coils 22, top pole 40, top pole track width 41, and a back gap area 42 where top pole 40 contacts bottom pole 20 (FIG. 3) are shown. As stated above, one way to write the servo erase pattern of FIG. 2 would be to utilize multiple write heads. However, because the write head is actually substantially wider than write track width 41 of top pole 40, the use of multiple heads would create a physical interference between the electromagnetic coils 22 of adjacent heads if the heads were placed relatively close together. If the heads were farther apart, then the write pattern of each would also be spaced farther apart requiring servo band 13 to be wider thus reducing the available tape area for data tracks 12. In addition, the use of multiple heads would require multiple write drivers which would have to be precisely synchronized to ensure a correct servo pattern. It would be desirable to have a tape head which could accurately write a sophisticated servo pattern including multiple erase patterns 16 such as is shown in FIG. 2 while keeping the head manufacture cost effective and allowing the pattern to be efficiently written onto a given width of tape in one write procedure.